The problem with really long chapters is that readers can find them daunting at times. For example, if the chapter is shorter then the reader is more likely to make decisions such as: "I'll just read this chapter and then go to bed". If the chapter is really long then a person is more likely to say: "Oh I can't be bothered reading all that." Your dedicated readers likely will be fine, but new readers are just as likely to skip it as read it. Maybe after the story has grown by a number of chapters, then you could slip in the odd long chapter, but certainly not at the start.
As far as writing for children, that's always a very difficult thing indeed, and one that a great many professional writers don't get right. I think as long as the child doesn't come away sounding too adult - and as long as the child is understandable - then you're fine. Observing how a real child of that age speaks is a good indicator.
